It is well-known in the metallurgical arts that the formation of metal foils using current methods is a complex, multi-step process typically involving various combinations of hot-working, cold-working, annealing and surface finishing. Foils are most frequently formed by a series of hot-rolling or cold-rolling steps, or some combination of both, on a previously formed metal sheet, the metal sheet itself resulting from prior forming operations performed on even larger bodies such as plates,slabs or billets, or in some cases from the output of a continuous casting process. The formation of metal foils from the class of alloys suitable for high temperature, high strength applications, such as Ti-base, Ni-base, Nb-base superalloys, is known to be particularly difficult and thus expensive, because of the elaborate processes required to produce these foils as described briefly below.
One related art method by which foils have been formed from some Ti-base alloys is by hot-working a sheet of the alloy to reduce the thickness, followed by surface grinding the sheet to a thinner dimension, followed still by chemical milling to achieve a final foil thickness. This method is limited in that it is very expensive due to the substantial material loss during the various processing steps, in addition to the costs associated with the processing steps themselves. Also, to the extent that the hot-working steps of this process may cause grain growth and/or grain texturing within the sheet, such features are frequently undesirable.
A second related art method of forming foils of Ti-base alloys, such as titanium aluminide, involves plasma spraying a pre-foil using alloy powder. The pre-foil is subsequently processed by steps such as roll consolidation, cold-rolling and annealing to form a foil having a typical thickness of about 0.003 in. This method also has several inherent limitations. One limitation is that the pre-foils formed by spray forming are not fully dense (i.e. near theoretical density) in that microscopic examination of them reveals internal porosity, such that additional consolidation is typically required. A second limitation is the strong propensity for many reactive alloys such as Ti-base alloys, to absorb oxygen, nitrogen or other contaminants during the plasma spray process used to form the pre-foil, even when the deposition is done in an an evacuated chamber which has been backfilled with argon. For example, Applicants have observed that an alloy powder of Ti-6Al-2Sn-4Zr-2Mo, with measured average oxygen and nitrogen levels of approximately 850 ppm O and 100 ppm N, produces an RF plasma-sprayed pre-foil having measured average levels of these elements of approximately 1950 ppm O and 140 ppm N. Similarly, in a Ti-14Al-21Nb alloy, Applicants measured average concentrations of oxygen and nitrogen of approximately 800 ppm O and 80 ppm N in the powder, as compared to average concentrations of 1350 ppm O and 160 ppm N in a pre-foil made from the same powder by plasma spraying.
A third related art method for forming metal alloy foils is described in U.S. Pat. No. 4,917,858. This patent describes a method for making titanium aluminide alloy foils by blending powders of elemental titanium and elemental aluminum in preselected proportions corresponding to desired Ti-Al alloy compositions. The blended elemental powders are hot-rolled at approximately 700.degree. C. to form a "green" foil having a thickness of 0.004-0.40 inches. The green foil is then sintered at 500.degree.-1200.degree. C. to form a foil having less than theoretical density. The sintered foil is then hot pressed at 800.degree.-1200.degree. C. to produce a finished foil having theoretical density. This patent also discloses the use of a third powder with the titanium and aluminum powders as an alloying element and lists niobium, molybdenum, vanadium, chromium, manganese, erbium, and yttrium as candidate third powder additives. However, this process requires at least three high temperature process steps which, due to the expense required to perform them, are known in the art to add significant cost to the final foil product. In addition, it is known that hot-rolling generally causes some degree of grain texturing or grain orientation, as well as having the potential to induce grain growth.